Starch is the principal carbohydrate constituent of all brewers' grains including malt, unmalted barley and non-malted cereal adjuncts, including corn and rice. The starch, regardless of the source, is a high molecular weight polymer consisting of glucose units joined primarily by alpha-1,4 glucosidic bonds but also including a smaller number of alpha-1,6 glucosidic bonds. The former type of bond occurs in the linear chains of glucose units while the latter type result in inter-chain branch points that are characteristic of a particular sub-group of starch known as amylopectins. Most starches comprise about 25% amylose and about 75% amylopectin.
In the brewing process, starch is first broken down during the mashing stages by alpha and beta amylases. As a result of this enzymatic activity, a spectrum of non-fermentable soluble dextrins and fermentable sugars are formed. The majority of these dextrins are alpha-1,6 linked carbohydrates. The soluble starch hydrolysates are then extracted from the spent grains as a wort solution which can subsequently be fermented to produce beer. The fermentable carbohydrates such as glucose, maltose and maltotriose are fermented by the brewers' yeast to produce ethanol while the higher molecular weight dextrins, being non-fermentable, remain substantially unchanged in the final beer and contribute to its body and smoothness. The presence of these unfermentable, high molecular weight oligosaccharides is therefore acceptable, and even desirable, in the production of "regular" or conventional beers, despite the fact that they also contribute about four per cent, on a weight by volume basis, of carbohydrate. In the production of the so-called "light" beers, however, these non-fermentable or marginal dextrins represent a source of unwanted calories. A variety of processes exist, any one of which is at least nominally suitable for dealing with these non-fermentable carbohydrates in a manner which ultimately reduces the number of calories per unit volume in a fermented wort. One such process involves the breakdown of the carbohydrate into fermentable sugars through the enzymatic hydrolysis of the alpha-1,4 and alpha-1,6 glucosidic linkages. By means of such a process the non-fermentable carbohydrates may be effectively and economically utilized towards the production of ethanol.
One process relating to the production of low dextrin beers is disclosed in U.S. Pat. No. 3,379,534. That process is facilitated through the use of an amyloglucosidase(syn.,glucoamylase)-containing preparation which acts to hydrolyse the alpha-1,4 and alpha-1,6 linkages which are characteristic of the marginal dextrins. In accordance with the teachings of that patent, this enzymatic activity reduces the residual dextrin content which permits a reduction of the caloric content of the beer and also makes better use of the starch materials by converting a larger proportion thereof into fermentable sugars. The amyloglucosidase-containing preparation is obtained as a culture filtrate from a fungal species, Aspergillus niger. This species of fungus has long been recognized as the principal source of commercial amyloglucosidase enzymes.
Amyloglucosidase derived from this fungal source and, indeed from any other known sources of such enzymes, suffer from a disadvantage attributable to these enzymes intrinsic thermal stability. Amyloglucosidase derived from Aspergillus niger species are commonly employed in the commercial production of starch hydrolysates such as dextrose syrups wherein the reaction millieu is typically held at about 60.degree. C. for periods of around seventy-two hours in duration. It will, of course, be appreciated that the rate of thermal denaturation of these enzymes in the pasteurization of beer is accelerated by the presence of ethanol in, and the acid pH of, the beer; however, that acceleration notwithstanding, it has been shown that such enzymes will not be adequately inactivated under the relatively mild pasteurization conditions to which beers are normally subjected. Thus, amyloglucosidases derived from Aspergillus niger culture filtrates and utilized in the hydrolysis of marginal dextrins in the brewing of beer must be employed prior to the kettle boiling of the wort, otherwise the enzymes survive in the finished, bottled product. The implications of that survival are three-fold: first, the enzyme will continue to act in the finished product on any remaining marginal dextrins to produce lower molecular weight fermentable carbohydrates such as glucose. Thus, at the very least, the beer will become a "sweet" beer. Secondly, if any viable yeasts or spores remain in the finished product, such organisms will act on the newly available fermentable carbohydrates and the finished product will undergo further fermentation. Either the sweetening of the beer and/or the fermentation of the newly formed carbohydrates will cause uncontrolled, and therefore undesirable, changes in the finished product after it has left the brewery. This aspect of the amyloglucosidase problem is briefly alluded to in an article appearing in the September, 1982 issue of "Brewers' Digest", beginning at page 14 thereof. This article, entitled "Enzymes in Brewing" by Marschall et al, suggests that the use of amyloglucosidase in the production of light beer, in view of the problems currently attendant such use will most likely depend upon the commercialization of immobilized enzyme technology whereby the amyloglucosidase enzymes will not be retained in the treated beer. Thirdly, brewing apparatus may become contaminated with the thermally stable enzyme which may result in "regular" beers becoming contaminated and detrimentally modified. The thermal stability of the enzyme may complicate its eradication from contaminated equipment. These problems are all addressed in an article by I. C. Wilcox et al, appearing in the MBAA Technical Quarterly, Vol. 14, No. 2, 1977 at page 105.
It is an object of the present invention to provide amylolytic enzymes derived from the yeast species Schwanniomyces castellii which enzymes are thermolabile under normal beer pasteurization conditions, relative to amylolytic enzymes derived from species of the genus Aspergillus.